Fouling can be defined as the accumulation of unwanted matter on heat transfer surfaces. This deposition can be very costly in refinery and petrochemical plants since it increases fuel usage, results in interrupted operations and production losses and increases maintenance costs.
Deposits are found in a variety of equipment: preheat exchangers, overhead condensers, furnaces, fractionating towers, reboilers, compressors and reactor beds. These deposits are complex; broadly, they can be characterized as organic and inorganic. They consist of metal oxides and sulfides, soluble organic metals, organic polymers, coke, salt and various other particulate matter. Chemical antifoulants have been developed that effectively combat fouling.
The chemical composition of organic foulants is rarely identified completely. Organic fouling is caused by insoluble polymers which sometimes are degraded to coke. The polymers are usually formed by reactions of unsaturated hydrocarbons, although any hydrocarbon can polymerize. Generally, olefins tend to polymerize more readily than aromatics, which in turn polymerize more readily than paraffins. Trace organic materials containing hetero atoms such as nitrogen, oxygen and sulfur also contribute to polymerization.
Polymers are generally formed by free radical chain reactions. These reactions, shown below, consist of two phases, an initiation phase and a propagation phase. In reaction 1, the chain initiation reaction, a free radical represented by R.sup..multidot., is formed (the symbol R.sup..multidot. can be any hydrocarbon). These free radicals, which have an odd electron, act as chain carriers. During chain propagation, additional free radicals are formed and the hydrocarbon molecules (R) grow larger and larger (see reaction 4), forming the unwanted polymers which accumulate on heat transfer surfaces.
Chain reactions can be triggered in several ways. In reaction 1, heat starts the chain. Example: when a reactive molecule such as an olefin or a diolefin is heated, a free radical is produced. Another way a chain reaction starts is shown in reaction 3. Here metal ions initiate free radical formation. Accelerating polymerization by oxygen and metals can be seen by reviewing reactions 2 and 3.
As polymers form, more polymers begin to adhere to the heat transfer surfaces. The hearing process results in dehydrogenation of the hydrocarbon and eventually the polymer is converted to coke.
1. Chain Initiation EQU R--H.fwdarw.R.sup..multidot. +H.sup..multidot.
2. Chain Propagation EQU a. R.sup..multidot. +O.sub.2 .fwdarw.R--O--O.sup..multidot. EQU b. R--O--O.sup..multidot. +R'--H.fwdarw.R.sup..multidot. +R--O--O--H
3. Chain Initiation EQU a. Me.sup.++ +RH.fwdarw.Me.sup.+ +R.sup..multidot. +H.sup.+ EQU b. Me.sup.++ +R--O--O--H.fwdarw.Me.sup.+ +R--O--O.sup..multidot. +H.sup.+
4. Chain Termination EQU a. R.sup..multidot. +R.sup..multidot. .fwdarw.R--R' EQU b. R.sup..multidot. +R--O--O.sup..multidot. .fwdarw.R--O--O--R
In refineries, deposits usually contain both organic and inorganic compounds. This makes the identification of the exact cause of fouling extremely difficult. Even if it were possible to precisely identify every single deposite constituent, this would not quarantee uncovering the cause of the problem. Assumptions are often erroneously made that if a deposit is predominantly a certain compound, that compound is the cause of the fouling. In reality, a minor constituent in the deposit could be acting as a binder, a catalyst, or in some role that influences actual deposit formation.
The final form of the deposit as viewed by analytical chemists may not always indicate its origin or cause. Before openings, equipment is steamed, waterwashed, or otherwise readied for inspection. During this preparation, fouling matter can be changed both physically and chemically. For example, water-soluble salts can be washed away or certain deposit constituents oxidized to another form.
In petrochemical plants, fouling matter is often organic. Fouling can be severe when monomers convert to polymers before they leave the plant. This can occur in streams high in ethylene, propylene, butadiene, sytrene and other unsaturates. Probable locations for such reactions include units where the unsaturates are being handled or purified, or in streams which contain these reactive materials only as contaminants.
Even though some petrochemical fouling problems seem similar, subtle differences in feedstock, processing schemes, equipment and contaminants can lead to variations in fouling severity. For example, ethylene plant depropanizer reboilers experience fouling that appears to be primarily polybutadiene in nature. The severity of this problem varies significantly from plant to plant, however. Average reboiler run length may vary from one to two weeks up to four to six months (without chemical treatment).
Although it is usually impractical to identify the fouling problem by analytical techniques alone, this information, along with knowledge of the process, processing conditions and the factors that contribute to fouling, are all essential to understanding the problem.
There are many ways, mechanical as well as chemical, to reduce fouling. Chemical additivies offer an effective means; however, processing changes, mechanical modifications equipment and other methods available to the plant should not be overlooked.
Antifoulants are formulated from several materials: some prevent foulants from forming, others prevent foulants from depositing on heat transfer equipment. Materials that prevent deposit formation include antioxidants, metal coordinators and corrosion inhibitors. Compounds that prevent deposition are surfactants which act as detergents or dispersants. Different combinations of these properties are blended to provide maximum results for different applications. These "polyfunctional" antifoulants are generally more versatile and effective since they are designed to combat various types of fouling that can be present in any given system.
Research indicates that even very small amounts of oxygen can cause or accelerate polymerization. Accordingly, antioxidant-type antifoulants have been developed to prevent oxygen from initiating polymerization. Antioxidants act as chain-stoppers by forming inert molecules with the oxidized free radical hydrocarbons, in accordance with the following reaction: